Moin Reece,
I stronlgy recommend to change the inlet filter. Would be no bad idea to change the second filter as well. And yes, itīs normal that it takes some pressure to make the air go through as the flow is not unrestricted.
I had a similar problem once with my KTM. Allthough it has a carby, the symptoms are similar. At lower revs the bike went just fine. But whilst quickly accelerating the power was instantly gone and the engine even stopped. The cause of this trouble was a bended rubber fuel pipe. It allowed just enough fuel to pass through to run the engine at low to medium revs but nothing that needed some power. The original fuel pipe in question got pinched and was replaced by me. But when I replaced it, I missed to look into the tiny tube. To avoid kinking a tiny metal spring had been inserted. After extracting it from the original and pressing it into the new feed pipe, the bending just followed the expected 'U' and didnīt get twisted or kinked again. This lack of fuel flow however may explain the whining of your fuel pump. Imagine the fuel pump being managed by the computer. It calculates the needed amount of fuel in regard to air and powersetting, donīt know whether the old ones take other values into consideration as well, and starts spinning accordingly to "produce" some fuel. But thereīs no fuel coming out of the feed pipe. What would you do, ... whine!
So, check the fuel supply and replace the filters. My 2 Eurocents,
DD
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